When searching for some content, the URL-Line in the browser shows only https://en.wikipedia.org/w/index.php and no search parameters can be found there. This is nasty, because when you go back in browser history you end up on the main page and not in the search results.
It seems to happen since about a week, so maybe by the update on May 24th/25th – maybe a week earlier.
Log in (this seems to be mandatory)
Go to https://en.wikipedia.org/w/index.php?search
Enter some search-text (I used "Marie") in middle of the page and press search
Everything is fine, the URL-line of the browser shows: https://en.wikipedia.org/w/index.php?search=Marie&title=Special%3ASearch&profile=advanced&fulltext=1&advancedSearch-current=%7B%7D&ns0=1
Press again search
Now the URL-line shows only https://en.wikipedia.org/w/index.php
If you cannot reproduce, expand that "Search in:"-field, checkmark "Remember selection for future searches", press search-button … wait for the result … press search-button again. From now on the search should behave like my description.